Short answer: calibration compares an instrument against a reference of known, traceable value and records the difference and its uncertainty; accuracy is how close a reading is to the true value, resolution is the smallest change it can display, and they are not the same thing.
What is calibration, and how does it work?
Calibration is a documented comparison between the instrument you use and a reference standard whose value is known with smaller uncertainty. The result tells you how far your instrument reads from the reference at each test point, and with what uncertainty.
Calibration does not necessarily mean adjustment. Adjustment changes the instrument so it reads closer to the reference; calibration records what it reads. A good certificate states results before and after any adjustment.
Traceability is the unbroken chain linking your measurement to national or international standards through a series of calibrations, each with stated uncertainty. In the UK, the National Physical Laboratory maintains national measurement standards, and UKAS accredits calibration laboratories to ISO/IEC 17025, the international standard for testing and calibration laboratory competence.
The internationally agreed vocabulary for these terms is published in the International Vocabulary of Metrology (VIM), and the method for evaluating uncertainty is the Guide to the Expression of Uncertainty in Measurement (GUM), both available from the BIPM.
What is the difference between accuracy, resolution and repeatability?
| Term | Meaning | Example question it answers |
|---|---|---|
| Accuracy | Closeness of a reading to the true value | Is 50.0 °C really 50.0 °C? |
| Resolution | Smallest change the display can show | Does it show tenths or hundredths? |
| Repeatability | Agreement between repeated readings under the same conditions | Do I get the same value three times? |
| Reproducibility | Agreement under changed conditions, such as different operators | Does a colleague get the same result? |
| Measurement uncertainty | A quantified range within which the true value is believed to lie | How confident can I be in this number? |
| Drift | Gradual change in readings over time | Is the instrument changing since last check? |
High resolution is easy to display; accuracy is harder to achieve. An instrument showing three decimal places can still be inaccurate.
How do I read an accuracy specification?
Many instruments state accuracy as a percentage of reading plus a number of counts or digits, for example "±(1% + 3 digits)". The percentage part grows with the reading; the digits part is a fixed amount in the last displayed digit. Some instruments state accuracy as a percentage of full scale, which means relative error is larger at the low end of the range.
Check the conditions attached: temperature range, warm-up time, measuring range and the period over which the specification holds. Outside those conditions the stated accuracy may not apply.
How often should instruments be calibrated?
There is no single interval for every instrument. Intervals depend on how critical the measurements are, how heavily and harshly the instrument is used, its stability history, the manufacturer's recommendation and the requirements of any quality system or customer. A common approach is to start with the manufacturer's suggested interval and adjust it based on calibration results over time: shorten it if instruments are found out of tolerance, and lengthen it cautiously if they are consistently stable.
Between calibrations, simple intermediate checks — measuring a reference block, buffer solution or test weight — catch problems early.
Where does calibration matter in the UK?
Instruments used for legal purposes, such as weighing for trade and certain measuring instruments, are subject to legal metrology rules, including the Non-automatic Weighing Instruments Regulations 2016 and the Measuring Instruments Regulations 2016. Quality management systems commonly require calibrated equipment for measurements affecting product quality. Food businesses check thermometers, laboratories verify balances and pH meters, electricians maintain test instruments, and engineering workshops calibrate micrometers and gauges.
What are the common calibration mistakes?
- Confusing resolution with accuracy.
- Calibrating outside the range you use. Choose test points across your working range.
- Ignoring uncertainty on the certificate when deciding pass or fail.
- Using expired or contaminated reference solutions for pH or conductivity.
- No records. Without a history, you cannot set sensible intervals.
- Assuming a new instrument needs no checks.
What can I do between calibrations?
Run quick checks at a fixed frequency against a known reference and record the results, so a trend is visible long before a formal calibration is due. Label instruments with their calibration due date.
Frequently asked questions
Is a calibration certificate the same as a UKAS certificate?
Not necessarily. A UKAS-accredited certificate comes from a laboratory assessed against ISO/IEC 17025 for that measurement. Other certificates may still be useful but do not carry accreditation.
Can I calibrate my own instruments?
You can carry out checks against references you hold, and for many purposes that is sufficient. For formal traceability, your references themselves need calibration.
What does traceable calibration mean?
That the result is linked through a documented chain of calibrations, each with stated uncertainty, to national or international standards.
Why does accuracy matter more than resolution?
Resolution only shows how finely a value is displayed. Accuracy tells you whether that value is correct.
What should a calibration certificate include?
Typically: instrument identification, test points, reference values, readings, measurement uncertainty, environmental conditions, date and the laboratory's details.
What should I check before ordering?
Before you order any instrument, write down the measurements or tasks it must handle, the range and accuracy you need, where it will be used and who will use it. Compare those notes with each product's specification sheet rather than its headline features, and keep the manual and any certificates with the instrument once it arrives.
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